1. Field of the Invention
This invention relates generally to a polyaxial pedicle screw and, more particularly, to a screw for insertion into spinal bone having a polyaxial coupling and locking mechanism for mounting a stabilizing rod to a sequence of vertebrae.
2. Description of the Prior Art
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. For the purposes of this disclosure, however, the word spine shall refer only to the cervical region.
Referring now to FIGS. 1, 2, and 3, top, side, and posterior views of a vertebral body, a pair of adjacent vertebral bodies, and a sequence of vertebral bodies are shown, respectively. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 includes a rearwardly and downwardly extending portion called the spinous process 16, and laterally extending structures which are referred to as the transverse processes 14. The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. The pedicles 24 comprise bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles.
"Rod assemblies" generally comprise a plurality of such screws which are implanted through the posterior lateral surfaces of the laminae, through the pedicles, and into their respective vertebral bodies. The screws are provided with upper portions which comprise, or have mounted thereto, coupling elements for receiving and securing an elongate rod therethrough. The rod extends along the axis of the spine, coupling to the plurality of screws via their coupling elements. The rigidity of the rod may be utilized to align the spine in conformance with a more desired shape.
It has been identified, however, that a considerable difficulty is associated with inserting screws along a misaligned curvature and simultaneously exactly positioning the coupling elements such that the rod receiving portions thereof are aligned so that the rod can be passed therethrough without distorting the screws. Attempts at achieving proper alignment with fixed screws is understood to require increased operating time, which is known to enhance many complications associated with surgery. Often surgical efforts with such fixed axes devices cannot be achieved, thereby rendering such instrumentation attempts entirely unsucessful.
The art contains a variety of attempts at providing instrumentation which permit a limited freedom with respect to angulation of the screw and the coupling element. These teachings, however, are generally complex, inadequately reliable, and lack long-term durability. These considerable drawbacks associated with prior art systems also include difficulty properly positioning the rod and coupling elements, and the tedious manipulation of the many small parts in the operative environment.
One such inferior solution is provided in the TSRH.sub..TM. spine system, produced by Sofamor Danek.sub..TM.. This product, illustrated in FIGS. 4a and 4b, comprises a fixed-axis screw 40 having a threaded shaft 42 and a pair of upwardly extending members 44a,44b which define a U-shaped upper portion 45. The front and back faces 46a,46b (back faces not shown) of the upwardly extending members 44a,44b are splined with a radially extending ridges 48.
Referring now to FIG. 4b also, a fixed cross bar member 50, which is mounted in the trough of the U-shaped upper portion 45 and transverse to the overall axis of the screw 40, is shown. The cross bar member 50 includes a first end 52 which is threaded, onto which a nut 54 may be advanced. The second end 56 of the cross bar member 50 comprises a hole 58 through which a rod may be disposed. The diameter of the first end 52 is less than the diameter of the second end 56. An annular member 60 is slideably mounted about the middle of the cross-bar member (at the junction of the first and second ends) which has opposing faces 62 and 64. The first face 62, which addresses the first end 52, has a splined conformation, such that it may join with the splined faces 46a,46b of the U-shaped section of the screw 40 (the splining permits a rotational variety of interfacing angles). The second face 64 includes a groove 66 in which the rod may nest when it is positioned through the hole 58. When the first end 52 of the cross bar member 50 is positioned in the U-shaped trough, and the nut 54 is advanced, the first face of the disc 60 locks to the splined faces 46a,46b of the screw 40, by virtue of the mutual splines. The annulus 60 itself is then slideably pushed toward the second end 56 of the cross-bar member 50 until the second face 64 of the annulus, and more particularly the groove 66, compresses the rod in the hole 58 of the cross bar member 50. This tightening of the nut 54, therefore, locks the assembly together.
The TSRH.sub..TM. spine system requires a multitude of cross bar lengths to account for different anatomies, which control the distance of the rod from the screw head. In addition, the fixed nature of the head of the screw severely limits the freedom of the surgeon to angulate the screws and to rotate the cross bar relative to the shaft portion. (The cross bar can only be rotated into a limited number of positions--all of which are transverse to the head of the screw. In a fixed axis screw system, this means the cross bar is always transverse to the shaft.) This limitation requires that screws be left higher--more "proud"--than would otherwise be comfortable for the post operative patient.
It is, therefore, the principal object of the present invention to provide a pedicle screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception.
In addition, it is an object of the present invention to provide such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation.
Accordingly it is also an object of the present invention to provide an assembly which is reliable, durable, and provides long term fixation support.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.